HELP...with downgrading from 3.40 to 3.40OE-A

i have 3.40 firmware on my psp. i have heard that the 3.40OE-A is a good firmware to use to play ISOs etc. can anyone tell me how to change my firmware to this? and also any loader programs i can use to load my ISO psp games up? i also want to be able to use or go back to the official sony psp updates if needed at any point. when down grading will this mess it up not being able to update again? another q is will i be able to use the GBA SNES and all the other type of system emulators out there with this firmwire.

thanks

sorry for all the questions, just so many to ask because of all the stuff there is out there.